The invention relates generally to mechanisms for adjusting the shape of a backrest, and more particularly, to mechanisms with lumbar baskets which conform more closely to the curvature of the human spine and to methods of constructing such mechanisms.
Mechanisms insertable into a backrest to adjust shape and provide better lumbar support are known. Examples are found in U.S. Pat. Nos. 5,050,930 and 5,397,164 to Schuster et al.
Prior art mechanisms comprise a shaping element, commonly referred to as a xe2x80x9clumbar basketxe2x80x9d, which is mounted for displacement along a guide track. The lumbar basket may have various configurations. A basic construction involves a pair of brackets displaceable along an axis of the guide track, resilient axial ribs joining the brackets and resilient transverse ribs fixed centrally to the axial ribs with free ends extending laterally to either side of the axial ribs to provide a cushioning effect. Various mechanisms can be used to draw the brackets together in order to flex the lumbar basket from a relative flat rest state to various bowed states. Various mechanisms can also be used to displace the lumbar basket axially along the track. Thus, the curvature of the lumbar basket and its position within a backrest can be adjusted to provide greater comfort.
The basic lumbar basket described above has a flexed profile which is essentially a sediment of a circle, and consequently does not conform adequately to the curvature of a user""s spine. A prior art approach to altering the flexed profile involves fixing a partial central rib to an upper bracket and an upper set of the transverse ribs, making the upper end of the basket more rigid. This induces greater flexing of the basket proximate to the lower bracket, providing greater comfort for many users. There are, however, shortcomings to such an approach. Making the partial rib and then fastening it to multiple components of the basic lumbar basket contributes to cost. There is also little freedom to specify the profile ultimately presented by the basket Various alternatives can be envisaged to produce baskets that flex to various profiles; however, the basic prior art lumbar basket is simple, and it would be desirable to avoid introducing components and manufacturing steps.
In one aspect, the invention provides a shape-adjusting mechanism insertable into a backrest to present a desired profile. The mechanism includes a lumbar basket which has a pair of supports spaced-apart along an axis. One or more resilient axial ribs are fixed to the supports so that the axial ribs flex in a predetermined direction as the supports are displaced toward one another, and resilient traverse ribs are fixed to the axial ribs. Means are provided to displace the supports to flex the axial ribs. To control flexing, each axial rib is structured to have lengthwise sections with different resistance to bending in the predetermined direction. For example, each axial rib may have a lengthwise section of relatively low bending resistance proximate to a selected support and another lengthwise section of relatively high bending resistance proximate to the other support The flexing of the axial ribs is thus more pronounced proximate to the selected support, and the lumbar basket may be installed in a backrest with the selected support lowermost where the pronounced flexing better conforms to the curvature of a user""s spine. More generally, lengthwise sections of different bending resistance can be located along each axial rib to achieve various preselected profiles which are felt to be appropriate. This approach eliminates the need to make additional components, such as partial stiffening ribs, and to affix such additional components to the basket.
Each axial rib is preferably stamped with its sections of different bending resistance formed during stamping. Although an axial rib may for such purposes be stamped with various cross-sections selected to impart different rigidity to different lengthwise sections of the rib, a preferred approach is to stamp each axial rib as an elongate strap with transverse reinforcing flanges bent along its lateral edges. The reinforcing flanges can be inclined at various angles relative to the general plane of the strap to select the relative structural rigidity of the different lengthwise sections. Apart from providing profile control without having to make additional stiffening components, the process involves fewer manufacturing steps than required for a basic prior art lumbar basket reinforced with one or more partial ribs.
Other aspects of the invention will be apparent from a description below of preferred embodiments and will be more specifically defined in the appended claims.